For over 160 years, anesthetics have been given safely and effectively to minimize the otherwise deleterious side effects of major invasive procedures. Despite such success, their exact mechanism remains elusive. This is confounded by the fact that our understanding of the conscious state which general anesthesia alters is grossly inadequate. However, over the past years, great insights have been gained into the molecular underpinnings of anesthetic mechanisms, just as they have with comparable factors within the central nervous system. It is now believed that anesthetics mediate a significant portion of their activity via binding to and modulation of transmembrane ligand-gated ion channels (LGICs). In particular, the gamma-aminobutyric acid receptor type A (GABAaR) and the glycine alpha one receptor (GlyRa1) are ion channels whose inhibitory currents are potentiated by the presence of general anesthetics. It is the GABAaR whose perturbation by barbiturates and benzodiazepines produces a state strongly similar to general anesthesia. Likewise, the intravenous general anesthetics, propofol and etomidate, are thought to modulate consciousness through specific sites within these channels. Jurd et al. (2003) Faseb J 17:250-2. These ion channels are composed of five subunits, often heteropentameric and variable in stoichiometry, arranged around a central ion conducting pore. The extracellular domain of a representative subunit is characterized by a large component of beta sheet secondary structure and contains the binding site for the native ligand germane to the channel in question (i.e. gamma-aminobutyric acid (GAB A) for the GABAaR). The transmembrane domain is composed largely of four-helix bundles. This latter secondary structure is a significant feature predicted in the inventors' laboratory and later validated by experimentalists. Bertaccini et al. (2002) Protein Eng 15:443-54; Miyazawa et al. Nature 424:949-55. It is within this transmembrane region that anesthetics and alcohols are thought to bind and convey the majority of their channel modulation. Bertaccini et al. (2010) J Chem Inf Model 50:2248-55.
The LGICs are transmembrane proteins, and their isolation and purification has thus proven difficult. While a recently published crystal structure shows the homomeric GABAaR beta 3 in the desensitized state (Miller et al. (2014) Nature 512:270-5) (a state that is insensitive to most anesthetics and alcohols), high resolution crystal structures of the heteromeric GABAaR in the open state (a state that anesthetics are thought to stabilize) do not exist. However, molecular modeling approaches are beginning to provide significant progress towards an understanding of LGIC structure and the interactions of these proteins with anesthetics.
The mainstay of such calculations lies in the techniques of homology modeling. Homology modeling is the method by which the amino acid sequence of a protein of unknown structure is aligned and threaded over that of a closely related amino acid sequence with known three-dimensional structure, such that the coordinates of the known protein can be transferred to those of the unknown. While such homology modeling involves a great deal of computational theory, it is also very dependent on experimentally described coordinates of proteins to act as templates with high sequence homology to the desired protein. Bertaccini et al. (2010) J Chem Inf Model 50:2248-55; Murail et al. (2011) Biophys J 100:1642-50. Over the last several years, several templates with great homology to the LGICs have been determined via cryo-electron microscopy, X-ray crystallography, and nuclear magnetic resonance (NMR) that have made model construction more robust.
The inventors' new models, based on such a template, can account for much of the currently available experimental data concerning these channels, allowing correlation of ligand binding measures with experimental potencies. These models have been used to illustrate the mechanism of channel gating (Bertaccini et al. (2010) ACS Chem. Neurosci. 1:552-558; Szarecka et al. (2007) Proteins 68:948-60) and can be used as the bases for in silico high throughput screening and new anesthetic discovery, and, in particular, to identify lead compounds for further screening in animals.
An April 2010 statement by MarketResearch.com reported that the world anesthetic drug market is estimated to be $4.1 billion. The drug profiles and dangerous side effects of the currently available agents are many, however, especially amongst the aging population of geriatric patients. Proportionately, the fastest growing segment of the population are octagenerians and older, a group of people who are now responsible for the majority of healthcare expenditures and are in need of increasing surgical and anesthetic care. Therefore, there is significant clinical pressure as well as market opportunity to develop new anesthetic agents.
There are currently four main intravenous and three primary inhalational anesthetic agents which are in common clinical use. Each of these agents is associated with an entire spectrum of undesirable hemodynamic perturbations, most of which result in lower systemic blood pressure. This is a side effect that is poorly tolerated in the very young patient, with newly developing cardiovascular compensatory mechanisms, as well as in the elderly, with confounding comorbidities and otherwise exhausted compensatory mechanisms.
Each of the commonly-used anesthetic agents has additional unique detriments. In particular, etomidate is the agent which comes closest to achieving ideal cardiovascular preservations while inducing dose-dependent alterations in consciousness, but this is at the expense of clinically significant adrenal suppression via inhibited steroid biosynthesis. U.S. Pat. No. 8,557,856 describes etomidate analogues having alleged improved pharmacokinetic and pharmacodynamics properties. U.S. Pat. No. 8,765,973 describes etomidate analogues that allegedly do not inhibit steroid biosynthesis. None of these etomidate analogues has yet achieved clinical approval, however.
Propofol has gained wide popularity, but in the past few years has been the subject of a nationwide shortage and can produce profound hypotension. While no profit estimates can be given for new anesthetic agents at this time, the cost savings from greater titratability and minimal cardiopulmonary side effects, both of which should lead to shorter and less complicated hospital stays, would be substantial.
There is thus a need for improved anesthetic agents, particularly for use in very young patients, elderly patients, and in patients that are critically ill. There is likewise a need for pharmaceutical compositions containing the improved agents and for methods of treatment that involve administration of the agents to induce or maintain anesthesia.